Previous correlational research has shown that the size of children's receptive and productive vocabularies correlates significantly with specific dimensions of their temperament. Infants and toddlers who are short-attending, socially unapproachable, negative in mood, and relatively distressed in response to novelty tend to demonstrate vocabulary delay. However, it is inappropriate to argue for a temperament-to-vocabulary causal direction of influence based on correlational data alone. In the proposed study, potential impacts of children's temperament during on-line vocabulary acquisition are examined under more tightly controlled environmental conditions. In a vocabulary acquisition task, children will be asked to fast-map novel words to novel objects under four environmental conditions. Each of the four conditions is designed to reflect different kinds of environmental stimulation that might normally be observed during bouts of word-learning. In a baseline condition, children's performance will be assessed when extraneous environmental stimulation is held to a minimum. Performance in this condition will be compared to performance under three distraction conditions: 1) in the presence of a large number of distracter stimulus items; 2) in the presence of a sudden-onset, moving, noise-making mechanical toy; and 3) in the presence of the sudden emergence of a strange adult who either 1) addresses the child by name and starts reading the Spanish version of a popular children's book, or 2) sits next to the child while looking and smiling at the child while providing no verbal stimulation. If children's temperament interacts with the environment to influence their ability to acquire words, then word-learning performance in a fast-mapping procedure should vary as a function of the interaction of temperamental profile and the types of competing information in the immediate environment. Results should provide evidence of underlying mechanisms linking dimensions of temperament to language delay.